Should Inter-Prosthetic Screws Be Placed When Prophylactically Plating a Femur?

BACKGROUND: Apposing intramedullary femoral implants can increase the risk of an interprosthetic femoral fracture. The aim of this study was to determine optimal screw configuration when prophylactically plating a femur with an interprosthetic segment of bone.

METHODS: Intramedullary implants were placed antegrade and retrograde to create two- and 10-cm inter-prosthetic distance (IPD) at the middiaphysis of 21 composite femora. Broad, curved, 18-hole 4.5 mm plates were applied to testing groups. Either one or zero screws (IP-1 or IP-0, respectively) were placed in the inter-prosthetic segment of bone. Femora were loaded to failure under lateral, four-point bending. We used one-factor analyses of variance with post hoc Tukey tests to compare the force-at-failure (FAF) and energy-to-failure (ETF). Digital image correlation was used to determine the surface strain distributions.

RESULTS: The FAF was greatest for 2- and 10-cm IP-0 (4,831 ± 213 and 4,437 ± 234 N, respectively) compared to all other conditions (P < 0.001). The ETF of 2- and 10-cm IP-0 (19.97 ± 1.15 and 23.57 ± 4.18 J, respectively) was significantly greater than the ETF of 10- and 2-cm IP-1 (5.89 ± 0.89 and 7.76 ± 2.59 J, respectively; P < 0.0004). The FAF and ETF were similar between nonplated native femora and those with 10-cm IPD (P > 0.06). Strain curves and heat mapping illustrated the concentration of strain at the screw tip exiting the far medial cortex.

CONCLUSIONS: A bicortical screw between apposing intramedullary femoral implants when prophylactically plating a femur creates a stress riser resulting in significantly lower FAF and ETF, regardless of inter-prosthetic distance. Femora with 10 cm or greater IPD have similar biomechanical properties to native femora. We suggest that bridging constructs, when prophylactically plating femora with apposing intramedullary implants with IPD less than 10 cm, mitigate the risk of interprosthetic femoral fracture.